One of the key aspects of SMPS (switched mode power supply) design may be seen in a reduced power consumption during standby mode, i.e. in an operation mode during which no load is connected to the SMPS, as well as during light load operation. Light load operation may be typically defined as an operation mode, during which a load is connected to the SMPS which corresponds to 5% or less of the nominal load of the SMPS.
A reduced power consumption and an increased efficiency during light load operation may be achieved by operating the SMPS in a burst mode. The burst mode is characterized by an on-phase usually lasting a few operating clock cycles during which the SMPS is in normal operation mode converting energy via a transformer and by an off-phase during which at least the power switches of the SMPS are turned off and no energy conversion takes place. In common SMPS various functionalities need to be upheld during the off-phase in the burst mode such that a significantly reduced power consumption of the SMPS during standby is not viable. For example, on the secondary side of the power converter an amplifier needs to be active constantly in order to determine the current load condition and transmit information with regard to the load condition to the primary side of the power converter. Thus, a receiver needs to be active on the primary side of the power converter in order to receive the information with regard to the load condition. Consequently, those and other control functions which need to remain active or be constantly upheld on the primary side and the secondary side of the power converter impose limitations on a possible reduction of the power consumption during the operation of the SMPS in burst mode, i.e. during standby. The burst mode is further limited by situations in which power consumption on the primary side is larger than the power consumption on the secondary side and the distribution of energy on the primary side and the secondary side during burst mode cannot take place according to need and the supply of energy on the primary side breaks down.
In further applications, SMPS are provided in which one optocoupler is used for transmitting a control signal and information about the burst mode from the secondary side to the primary side, wherein the receiving side of the optocoupler is provided with energy from one energy source, for example from a 5 V potential via a 23 kOhm resistor. In such a configuration, the current provided to the optocoupler tends to be too low as to prevent parasitic capacitances from affecting the signal transmission bandwidth during normal operation mode and it tends to be too high as to enable a further reduction of the overall power consumption of the SMPS during burst mode, which may be in the range of 0.5 mA to 1 mA, for example. There are also SMPS available which use more than one optocoupler for implementing communication between the primary side and the secondary side of the SMPS.